Inverter Size Calculator
Size your inverter with surge capacity for motor startup — enter appliances, get the kVA rating.
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Inverter Size Calculator
Size your inverter with surge capacity for motor startup — enter appliances, get the kVA rating.
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Estimates based on stated inputs and standard derating. Real-world performance varies with temperature, age, cable quality, and charge profile. Always verify with manufacturer datasheets.
How to Size an Inverter
This inverter size calculator determines the correct inverter kVA rating for your loads. Enter each appliance’s wattage and identify the largest motor load (fridge, AC, pump). The calculator adds up continuous watts and factors in the motor startup surge — which is 3 to 5 times the running watts — to recommend the minimum inverter capacity that handles both normal operation and the heaviest startup event.
Undersizing your inverter is the most common and most expensive mistake in solar and backup systems. An inverter that trips on overload every time the AC compressor kicks in is not just annoying — repeated overloads damage the inverter’s power electronics and void most warranties. Oversizing by 25 to 50 percent above the calculated minimum is standard practice.
Inverter Size Quick Reference
| Load Scenario | Continuous Watts | Surge Watts | Inverter Size |
|---|---|---|---|
| Lights + router + phone charging | 100W | 100W | 300W |
| Above + fridge | 250W | 700W | 1 kVA |
| Above + TV + fans + laptop | 500W | 700W | 1.5 kVA |
| Above + washing machine | 900W | 2,200W | 3 kVA |
| Above + 1-ton AC | 2,100W | 5,200W | 5-6 kVA |
| Whole house with 2-ton AC | 3,500W | 8,500W | 8-10 kVA |
These are guidelines for standard residential loads. Your actual requirement depends on which appliances run simultaneously and the largest motor in your system. The calculator above accounts for all of this automatically. For a detailed explanation of inverter types, see our inverter sizing guide.
kVA vs Watts: What Inverter Ratings Mean
Inverters are rated in kVA (kilovolt-amperes), not watts. The relationship: Watts = kVA × 1,000 × Power Factor. Most inverter manufacturers use a power factor of 0.8, meaning a 5 kVA inverter delivers 4,000 watts continuously. A 3 kVA inverter delivers 2,400 watts. Always check whether the spec sheet lists kVA or watts — they are not the same number. Our kVA to Watts Converter handles this conversion at any power factor.
Frequently Asked Questions
What size inverter do I need for my house?
Add up the wattage of every appliance you want to run simultaneously, then multiply the largest motor load (AC, fridge, pump) by 3 for surge. A typical essentials-only backup (fridge + lights + fans + router) needs a 1.5 kVA inverter. Adding air conditioning brings it to 5-6 kVA. Use the calculator above for exact sizing based on your appliance list.
Can I use a smaller inverter with a soft starter?
Yes. A soft starter (like the Micro-Air EasyStart) reduces motor startup surge by 60-70%. A fridge that normally surges to 600W drops to 200W with a soft starter. This can allow a 1 kVA inverter where a 1.5 kVA would otherwise be needed. See our EasyStart review for details.
Should I oversize my inverter?
Yes, by 25-50%. An inverter running at 80% capacity constantly runs hotter and wears faster than one running at 50%. The cost difference between a 3 kVA and a 5 kVA inverter is typically $100-200 — a small price for headroom, future load additions, and longer equipment life.
Pure sine wave vs modified sine wave inverter?
Always choose pure sine wave for permanent installations. Modified sine wave inverters cause motors to run hotter, electronics to buzz, and some devices (medical equipment, laser printers, variable-speed tools) to malfunction. The price difference has narrowed significantly — a 3 kVA pure sine wave inverter costs only $50-100 more than modified. See our hybrid inverter comparison for recommendations by use case.